Somatotropin is a polypeptide secreted by the anterior pituitary in animals and acts through specific cell surface receptors located primarily in the liver (Bibliography 1). Recently, a protein which binds somatotropin with high affinity has been identified in serum (2,3,4). This serum somatotropin binding protein closely resembles the molecules of the somatotropin receptor present on the surface of target cells and a large body of evidence has suggested the colinearity and identity between the extracellular domain of the somatotropin receptor and the somatotropin binding protein (5,6,7,8,9,10,11). Monoclonal antibodies have been raised to the somatotropin receptor which cross-react with somatotropin binding protein (12). It has also been proposed that the somatotropin binding protein is a product of proteolytic cleavage of the somatotropin receptor (9,11,13).
Recently, two species of somatotropin-related mRNA's in rats have been identified and cDNA cloned (14). One mRNA encodes a membrane bound somatotropin receptor and the other encodes the somatotropin binding protein which is a secreted soluble protein identical to the extracellular region, but lacking the transmembrane and intracellular domains of the somatotropin receptor. This extracellular region is replaced by an alternative splicing mechanism with a 17 amino acid hydrophilic segment followed by a unique 3'untranslated region. Thus, the somatotropin binding protein in rat has a unique carboxyl terminus which is absent in the somatotropin receptor. A similar molecular structure has also been reported with mouse somatotropin binding protein (15).